Daily fetal and maternal plasma samples collected from eight chronically catheterized pregnant ewes from day 130 of gestation through delivery were analyzed for progesterone (P), 20α-dihydroprogesterone [20α-hydroxy-4-pregnene- 3, 20-dione (20P)], 17α-hydroxyprogesterone [17α-hydroxy-4- pregnene-3, 20-dione (17P)], and 11-desoxycortisol [17α, 21-dihydroxy- 4-pregnene-3, 20-dione (Cp S)] by RIAs rigorously validated for specificity in this species. Fetal and maternal P and 20P concentrations fell with advancing gestational age and approaching parturition, while 17P and Cp S concentrations rose. Exponential type regressions for the logarithm of fetal and maternal concentrations of P, 20P, 17P, and Cp S for each animal were plotted as a function of days before parturition. Estimates were made from each fitted curve of the time (T*) for each animal at which changes in P and 20P concentrations began to occur. Mean T* ±SEM values for fetal and maternal P and 20P were: fetal P, −2.7 ± 0.5 days; fetal 20P, -3.0 ± 0.3 days; maternal, −3.5 ± 0.5 days; and maternal 20P, −3.4 ± 0.3 days. Similar time-trend effects suggest a common regulatory focus for P and 20P production. P concentrations were markedly higher in maternal than fetal circulation, while the reverse was found for 20P. This finding is consistent with a highly active fetal 20α-reductase system. Analysis of 17P produced exponential-type curves of nonuniform shape, strikingly different from P and 20P. These curves indicate different time-trend effects and imply that other regulatory foci are involved in 17P production. Exponential-type curves for Cp S in both fetal and maternal circulations were either concave upward or positive sloping straight lines, a finding which may reflect the adrenal origins of this steroid.
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